From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id CF76FC47DD9 for ; Wed, 28 Feb 2024 12:33:25 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rfJ75-0001u4-67; Wed, 28 Feb 2024 07:32:43 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rfJ6t-0001rG-KX for qemu-devel@nongnu.org; Wed, 28 Feb 2024 07:32:31 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rfJ6o-0008Rh-Vo for qemu-devel@nongnu.org; Wed, 28 Feb 2024 07:32:31 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1709123543; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=3phnP5v7W3Ds21SEGFmKyr0YeItnA0S46yr5ZB4aU9I=; b=Nqw+p8Hms8rW3oel1RD3llyG4whyecJxHEefVbaBISCYmDmSPE/17PlNGzHvSTzYOUYglE 8xFrAPiqaAYC640VFa3T1+lbmcAbA3yOmTnQoAXH0Mq5MiQmTGdHN580z064WFAuYtt4Xt zV6mOG112X9rtOJ/WfhPb9kP+kQGcDc= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-341-iQe1CziZPxOLKQpL3dJfDg-1; Wed, 28 Feb 2024 07:32:20 -0500 X-MC-Unique: iQe1CziZPxOLKQpL3dJfDg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id C3FF2185A784; Wed, 28 Feb 2024 12:32:18 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.193.4]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 9D931C185C0; Wed, 28 Feb 2024 12:32:18 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 73D3721E66F4; Wed, 28 Feb 2024 13:32:17 +0100 (CET) From: Markus Armbruster To: Stefano Garzarella Cc: qemu-devel@nongnu.org, Paolo Bonzini , qemu-block@nongnu.org, Daniel P. =?utf-8?Q?Berrang=C3=A9?= , Thomas Huth , "Michael S. Tsirkin" , Jason Wang , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , Eric Blake , Coiby Xu , slp@redhat.com, Eduardo Habkost , Hanna Reitz , Igor Mammedov , Raphael Norwitz , Kevin Wolf , David Hildenbrand , stefanha@redhat.com, gmaglione@redhat.com Subject: Re: [PATCH 9/9] hostmem-file: support POSIX shm_open() In-Reply-To: <20240228114759.44758-10-sgarzare@redhat.com> (Stefano Garzarella's message of "Wed, 28 Feb 2024 12:47:59 +0100") References: <20240228114759.44758-1-sgarzare@redhat.com> <20240228114759.44758-10-sgarzare@redhat.com> Date: Wed, 28 Feb 2024 13:32:17 +0100 Message-ID: <874jdswyzy.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.8 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.102, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Stefano Garzarella writes: > Add a new `shm` bool option for `-object memory-backend-file`. > > When this option is set to true, the POSIX shm_open(3) is used instead > of open(2). > > So a file will not be created in the filesystem, but a "POSIX shared > memory object" will be instantiated. In Linux this turns into a file > in /dev/shm, but in other OSes this may not happen (for example in > macOS or FreeBSD nothing is shown in any filesystem). > > This new feature is useful when we need to share guest memory with > another process (e.g. vhost-user backend), but we don't have > memfd_create() or any special filesystems (e.g. /dev/shm) available > as in macOS. > > Signed-off-by: Stefano Garzarella > --- > I am not sure this is the best way to support shm_open() in QEMU. > > Other solutions I had in mind were: > > - create a new memory-backend-shm How would that look like? Would it involve duplicating code? > - extend memory-backend-memfd to use shm_open() on systems where memfd is > not available (problem: shm_open wants a name to assign to the object, but > we can do a workaround by using a random name and do the unlink right away) Hmm. Too much magic? I don't know... > Any preference/suggestion? [...] > diff --git a/qapi/qom.json b/qapi/qom.json > index 2a6e49365a..bfb01b909f 100644 > --- a/qapi/qom.json > +++ b/qapi/qom.json > @@ -682,6 +682,9 @@ # @mem-path: the path to either a shared memory or huge page # filesystem mount Does this need adjustment? [...] > # writable RAM instead of ROM, and want to set this property to 'off'. > # (default: auto, since 8.2) > # > +# @shm: if true, shm_open(3) is used to create/open POSIX shared memory > +# object; if false, an open(2) is used. (default: false) (since 9.0) > +# Please format like this for consistency: # @shm: if true, shm_open(3) is used to create/open POSIX shared memory # object; if false, an open(2) is used (default: false) (since 9.0) > # Since: 2.1 > ## > { 'struct': 'MemoryBackendFileProperties', > @@ -692,6 +695,7 @@ > 'mem-path': 'str', > '*pmem': { 'type': 'bool', 'if': 'CONFIG_LIBPMEM' }, > '*readonly': 'bool', > + '*shm': 'bool', > '*rom': 'OnOffAuto' } } > > ## [...]